OBJECTIVE To examine the effect of latanoprost (50 and 500 ug/ml) solutions on hair regrowth in the bald scalp of stumptailed macaques RESULTS Lantanoprost (Xalatan) using treatment for glaucoma has a peculiar hypertrichotic side effect on eyelashes; hairs grow longer and thicker. We examined the effect of latanoprost on hair regrowth in the bald macaque. Latanoprost, 50ug/ml and 500ug/ml, 0.5 ml per day, were topically applied on the bald scalp of four monkeys for 3 to 8 months. Another four monkeys were given vehicle alone for 5 to 8 months. Evaluations were made by 1) monthly global photographs, 2) micromorphometry of follicular growth in biopsied scalp skin (folliculogram analysis), and 3) hair counts for the conversion rates of vellus hair to intermediary or to terminal types (phototrichography).The effects of hair and follicular regrowth were dose-dependent. Latanoprost (500ug/ml) induced significant increase of length and density of hairs, over 80% increase of follicular growth, and 27 to 74% increase of terminal hairs, in those compared to 0 time and 3 monthUs data. Xalatan (latanoprost, 50ug/ml) showed minimal degree of hair and follicular growth even by longer treatment period (8 months). Latanoprost, with its relatively low concentration (0.05%), markedly stimulated follicular regrowth in the macaque bald scalp. FUTURE DIRECTIONS To detect systemic side effects of this potent hair growth stimulator for clinical application of the treatment of baldness KEY WORDS latanoprost. Hair growth. Baldness. Stumptailed macaque FUNDING Pharmacia Upjohn Company